Method of maintaining the viscosity of cellulose esters uniform



Patented Oct. 31, 1939 METHOD OF MAINTAINING THE VISCOSITY OF C'ELLULOSE ESTERS UNIFORM Carl J. Malm, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application March 17, 1938, Serial No. 196,473

4 Claims.

This invention relates to the manufacture of cellulose esters of uniform viscosity by maintaining the temporary and permanent hardness of the wash water substantially the same as measured by their effect, at all times.

,- This application is a continuation-in-part of my application Serial No. 78,447, filed May '7, 1936, which has matured into Patent No. 2,126,488, dated August 9, 1938.

In the manufacture of organic acid esters of cellulose, such as cellulose acetate of a particulartype, the esterification is usually carried out for a time sufficient to give a selected first stage or acetic acid-tetrachlorethane viscosity. The ester is then hydrolyzed until the desired acetyl value is obtained. It has been observed how ever that even though a given type of ester exhibits the same first-stage viscosity at all times and seasons, the resulting product after hydrolysis, precipitation and Washing, varies in its viscosity in acetone from one time to the next. This variation has been compensated for by varying the firstestage viscosity but this results in esters in which the cuprammonium viscosities re not uniform. Variation of the first-stage viscosity has not been a satisfactory method of compensating for the variation in acetone viscosity, as the value for the first-stage viscosity depended on a prediction as to the final acetone viscosity, which prediction might be rather inaccurate.

I have found that the variation of the acetone viscosity of the cellulose ester is governed by the temporary and permanent hardness of the Water which is employed to wash the cellulose ester after its, hydrolysis. Substantially all large supplies of 'water are derived from surface water which varies in hardness, both temporary and permanent, depending on the prevailing conditions. For instance, the temporary hardness may be 0 at times and at other times be 40 P. P. M., 50 P. P. M. or more. The permanent hardness likewise may vary considerably from time to time. I have found that it is this variation in the hardness of the wash water that causes the variation in the acetone viscosity of partially hydrolyzed organic acid esters of cellulose prepared by the same' process.

One object of my invention is to prepare partially hydrolyzed organic acid esters of cellulose which exhibit a uniform acetone viscosity regardless of season or the prevailing conditions. Another object of my invention is to avoid any necessity of varying the first-stage viscosity of the esters of cellulose which was formerly thought to be necessary to assure uniformity of acetone viscosity. Other objects will appear herein.

I have found that if the hardness of the water employed for washing partially hydrolyzed cellulose esters is maintained constant, the acetone viscosity of the esters may be easily maintained uniform over as long a period of time as desired. The preferred method of maintaining constant hardness in the water to be employed for washing is to add calcium carbonate and calcium sulfate where necessary to maintain the water at the maximum hardness, both permanent and temporary throughout the entire year. An ester of uniform viscosity and stability may thus be obtained.

The temporary hardness of the Water to be employed is determined and if this value is below that which is to bev maintained at all times, wa-

ter containing sufficient bicarbonate of an alkad should be maintained uniform in order to get a constant acetone viscosity. This may be attained by repeatedly acylating celluloses of the same uniform quality and cuprammonium viscosity under the same conditions such as proportions, temperatures and times. The cellulose, such as refined cotton linters, may or may not be pretreated such as with acetic acid or with a mixture of acetic and sulfuric acids. For the higher viscosity esters, it is desirable to presoak with a catalyst-free bath essentially consisting of acetic acid to swell the cellulose prior to esterification. It is then esterified, such as by means of a bath consisting of acetic anhydride and a catalyst, such as sulfuric acid. The amount of acetic anhydride should be at least sufficient to completely .esterify the cellulose and to take care of any moisture present.

The temperature of esterification may be allowed to reach F., F. or more according to the first-stage viscosity desired. If the tem perature is to be restricted, it is desirable to cool the reacting materials before the reaction is started. In preparing a high viscosity ester, it is preferred that 6-8% of sufuric acid be used to speed up the reaction at lower temperatures and thereby minimize the lowering of the viscosity.

After the cellulose has been completely es-terified, as evidenced by its complete dissolution in the reaction mixture, it is hydrolyzed, such as by adding aqueous acetic acid with or without additional catalyst, as desired, and allowing to stand at a hydrolyzing temperature, usually F. until the desired acetyl content is obtained.

It is then isolated by precipitating in water or dilute acid and washing with water, in which the hardness has been standardized, until com- Example Over a period of years the fluctuation in water, utilized for washing cellulose acetate in temporary hardness, was 0-40 P. P. M. and in permanent hardness, was 50-120 P. P. M. To prepare cellulose acetate, having 40 seconds viscosity in acetone, it has been necessary to vary the firststage viscosities between 28 and 32 seconds, the finished'product averaging 40 seconds. Often it has been necessary to blend the products to obtain the average viscosity.

Since the temporary hardness of the water used to wash the cellulose acetate has been maintained at 40 P. P. M. and the permanent hardness at P. P. M., the first-stage viscosity has been kept at 28 seconds and a product, having an acetone viscosity of 40 seconds, has been consistently obtained.

The acetone viscosity is determined by the well-known falling ball method. Any method of determining the acetone viscosity may be used to check the uniformity of the viscosity of the cellulose ester prepared.

Both the temporary and the permanent hardness of the wash water has its effect on the viscosity of the cellulose ester, the former being more effective than the latter. Although the most practical way of carrying out my invention is to assure the same temporary hardness and the same permanent hardness'in the wash water of like type, contemplates temporary or predominantly permanent. To adjust the effective hardness it would be desirable to chart the effect of the salts on the particular cellulose ester in question and to determine from that chart the amount of calcium bicarbonate or some other alkaline earth metal salt to bring the efiective hardness up to the maximum used for a series of batches of the particular ester being prepared.

I claim:

1. In the manufacture of lower fatty acid esters of cellulose in which cellulose is acylated, partially hydrolyzed, precipitated out from the reaction materials, washed with water and dried, the method of maintaining uniform Viscosity of the esters which comprises maintaining the effective hardness of the water employed for washing the cellulose ester, uniform over an extended series of batches.

2. In the manufacture of lower fatty acid esters of cellulose in which cellulose is acylated,

partially hydrolyzed, precipitated. washed with I water and dried, the method of maintaining uniform viscosity of the esters which comprises maintaining both the temporary and the permanent hardness of the water employed for washing the cellulose ester, uniform over an extended series of batches.

3. In the manufacture of cellulose acetates in which cellulose is acetylated, partially hydrolyzed, precipitated, washed with water and dried, the method of maintaining uniform viscosity of the esters which comprises maintaining the effective hardness of the water employed for washing the cellulose acetate, uniform over an extended series of batches.

4. In the manufacture of cellulose acetates in which cellulose is acetylated, partially hydrolyzed, precipitated, washed with Water and dried, the method of maintaining uniform viscosity of the esters which comprises maintaining both the temporary and the permanent hardness of the water employed for washing the cellulose acetate, uniform over an extended series of batches.

CARL J. MALM. 

